Photodiodes with photoconductive gain enhancement

ABSTRACT

The present invention provides photodiodes exhibiting photoconductive gain. It is shown that photodiodes may exhibit photoconductive gain under certain conditions, and traditional photoconductive gain theory has been extended to describe these cases. Particularly, there is introduced the basic principles of photoconductive gain in p-i-n diodes, and there is described several approaches to designing photodiodes with photoconductive gain. In one approach, photogenerated carrier delay is used to obtain photoconductive gain in a photodiode. Delay structures inserted into the intrinsic region preferentially impede the flow of one of the carriers relative to the other to obtain the gain. Another method of obtaining photoconductive gain in a photodiode is to increase the rate at which electron-hole pairs are generated in the p-region or n-region, so as to decrease the times τ p  or τ n . One way of decreasing τ p  or τ n  is to include a region of “ultra-fast” high B-coefficient material in or near either the p-region or n-region, which has a much lower value of τ p  or τ n  for a given doping level.

This application claims the benefit of provisional application No.60/129,365, filed Apr. 15, 1999.

FIELD OF THE INVENTION

The present invention relates to photodiode devices exhibitingphotoconductive gain enhancement.

BACKGROUND OF THE INVENTION

Photodiodes are one of the most basic building blocks of photonicsystems, and the physics of p-i-n diodes is an area considered by manyto be well understood. Conventional treatment of a pi-n diode assumes amaximum photoconductive gain of unity, g=1. This assumption has provenso reliable for bulk photodiodes that it has become accepted as afundamental property of photodiodes, although the mechanisms preventinggreater than unity photoconductive gains have not been well described inthe literature. More recently, p-i-n photodiodes have been designed withheterojunction structures (such as multiple quantum well structures) inthe intrinsic region, and to date these devices have been modeled usingthe same assumption of unity photoconductive gain (for example see M. K.Chin and W. S. C. Chang, “InGaAs/InAIAs Quantum-Well ElectroabsorptionWaveguide Modulators With Large-Core Waveguide Structure: Design AndCharacterization”, Appl. Opt., 34:1544-1553, 1995; A. M. Fox, D. A. B.Miller, G. Livescu, J. E. Cunningham, and W. Y. Jan, “Quantum WellCarrier Sweep Out: Relation To Electroabsorption And ExcitonSaturation”, IEEE J. Quantum Electron., 27:2281-2295, 1991).

The ability to produce photodiodes with controlled photoconductive gainwould be of great benefit for use in photodetector devices and the like.For example, any application requiring detection of very low lightintensity levels would benefit significantly with the use of photodiodeshaving photoconductive gain. Therefore, it would be advantageous toprovide semiconductor photodiodes exhibiting photoconductive gain.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide photodiode devicesexhibiting photoconductive gain.

The inventor has demonstrated that photodiodes may exhibitphotoconductive gain under certain conditions, and that these gains maybe described to first order by a simple extension of photoconductivegain theory. In the present application there is introduced the basicprinciples of photoconductive gain in p-i-n diodes, and there isdescribed several approaches to designing photodiodes withphotoconductive gain.

In one aspect of the invention there is provided a photodiode withphotoconductive gain, comprising:

a semiconductor including at least one p-type region, at least onen-type region, and electrical contacts attached to said n-type regionand to said p-type region, whereby illuminating said semiconductorgenerates electron-hole pairs in at least one depletion region formed ata p-n junction; and

means for enhancing an electron-hole pair generation rate in at least aportion of said semiconductor outside said at least one depletionregion,

In this aspect of the invention the means for enhancing an electron-holepair generation rate may comprise the at least a portion of thesemiconductor including an effective semiconductor material having aminority carrier lifetime shorter than or on an order of a differencebetween the average times for an electron and a hole photogeneratedwithin a specific depletion region to escape the specific depletionregion.

In this aspect of the invention the means for enhancing an electron-holepair generation rate in at least a portion of the semiconductor outsidethe at least one depletion region may be by optical stimulation of theat least a portion of the semiconductor.

In another aspect of the invention there is provided a photodiode withphotoconductive gain, comprising:

a semiconductor including at least one p-type region, at least onen-type region, at least one intrinsic region, and electrical contactsattached to said n-type region and to said p-type region, wherebyilluminating said semiconductor generates electron-hole pairs in atleast one depletion region formed at a p-n junction and/or at anintrinsic region; and

means for enhancing an electron-hole pair generation rate in at least aportion of said semiconductor outside said at least one depletionregion.

BRIEF DESCRIPTION OF THE DRAWINGS

The method of generating photoconductive gain in photodiodes forming thesubject invention will now be described, by example only, referencebeing had to the accompanying drawings, in which:

FIG. 1a is an energy band diagram for a p-i-n photodiode illustrating anexample of carrier collection for a photogenerated electron-hole pairwhere photoconductive gain does not occur;

FIG. 1b is an energy band diagram of a p-i-n photodiode illustrating anexample of carrier collection for a photogenerated electron-hole pair inthe case where photoconductive gain does occur;

FIG. 2 shows experimental (symbols) and theoretical (lines)photoconductive gains for three InGaAs/InP p-i-n photodiodes as afunction of applied reverse bias voltage;

FIG. 3 shows a p-n photodiode produced in accordance with the presentinvention exhibiting photoconductive gain;

FIG. 4 shows a another embodiment of a photodiode according to thepresent invention designed to exhibit photoconductive gain; and

FIG. 5 shows an alternative embodiment of a photodiode exhibitingphotoconductive gain.

DETAILED DESCRIPTION OF THE INVENTION

THEORY

The inventor has unexpectedly discovered that photoconductive gaingreater than unity can be achieved in for example p-i-n photodiodes, andthat these gains may be described to first order by a simple extensionof photoconductive gain theory. This theory provides insight into themechanisms that prevent photoconductive gain in typical photodiodes, andat the same time provides a basis for the design of photodiodes withgreater than unity photoconductive gains.

Consider as an example a generic p-i-n photodiode where electronvelocities are greater than hole velocities in the depletion region, andwhere the electrical connections at both the anode and cathode exhibitohmic behavior. In order to simplify this initial description ofphotoconductive gain mechanisms in p-i-n photodiodes, assume that thephotogenerated electron and hole both survive long enough to becollected at the edges of the intrinsic region. On average, when anelectron-hole pair (EHP) is photogenerated in the depletion region, theelectron will be collected at the n-region (cathode) before the holereaches the p-region (anode), leaving a net positive charge in theintrinsic region of the photodiode. The resulting majority carrier flowthrough the n-region, external circuit and p-region will introduce anexcess electron in the valence band of the p-region, where iteffectively decreases the hole population by one. This depleted hole maynow be replaced by one of two mechanisms: either the photogenerated holewill be collected at the p-region; or EHP generation will restore theequilibrium hole population and provide an excess minority electron,which may then be injected into the depletion region.

These two mechanisms are illustrated in p-i-n photodiode energy banddiagrams FIGS. 1a) and 1 b) respectively, for a single EHP generated inthe intrinsic region, where the generated electron escapes the intrinsicregion before the generated hole does. Let us define t_(e) and t_(h) tobe the escape times for the electron and hole for this specific EHP, andt_(p) to be the time it takes for this specific excess electron in thevalence band of the p-region to be excited into the conduction band. Fort_(p)>t_(h)−t_(e), the photogenerated hole is collected at the p-regionbefore the p-region has time to supply an excess electron to theintrinsic region, thus preventing photoconductive gain.

In the second mechanism, EHP generation will restore the equilibriumhole population and provide an excess minority electron, which will thenbe injected into the depletion region. Referring to the energy banddiagram of FIG. 1b, if t_(p)<_(h)−t_(e), the excess electron will beinjected into the intrinsic region before the photogenerated holereaches the p-region, thus providing the mechanism for photoconductivegain.

Now let us consider the case where there are not one, but many EHPsgenerated in the intrinsic region, with a distribution of times t_(p)and t_(h)−t_(e). If the photogenerated holes remain in the depletionregion long enough, the depleted holes in the p-region will be replacedby EHP generation, as illustrated in FIG. 1b. Deviations in carrierpopulations from their equilibrium values decay exponentially in time,with a characteristic time constant called the recombination lifetime,or minority carrier lifetime, τ. Thus after an average time delay τ_(p),the excess electron in the valence band of the p-region will be excitedinto the conduction band, re-establishing the equilibrium holepopulation, and providing an excess minority electron in the p-region tobe swept into the depletion region. However, if the photogenerated holereaches the p-region first, as illustrated in FIG. 1a, this process willbe prevented, and no excess minority electron will be produced. Thecondition for photoconductive gain in a p-i-n photodiode may be writtenas

τ_(p) ≦t _(have) −t _(eave), for t _(have) >t _(eave),  (1)

and

τ_(n) ≦t _(eave) −t _(have), for t _(have) <t _(eave),  (2)

where τ_(p) and τ_(n) are the minority carrier lifetimes in the p-regionand n-region of the photodiode, and t_(eave) and t_(have) are theaverage times the photogenerated electron and hole spend the intrinsicregion. Note that these are qualitative conditions (the symbol “≦” asused herein means less than or on the order of) because τ_(p), τ_(n),and |t_(eave)−t_(have)| are only average values, and photoconductivegain can be expected to occur even in cases where τ_(p) and τ_(n) aremuch greater that |t_(eave)−t_(have)|, although with a low enoughprobability that the effect becomes negligible.

In practice, minority carrier lifetimes vary inversely with majoritycarrier concentrations, such that in general,

τ=1/(BN),  (3)

where N is the majority carrier concentration of the region of interest.(It has been assumed here that minority carrier concentrations arenegligible compared to majority carrier concentrations). B is acharacteristic coefficient for a given material, and is related to thethermal EHP generation rate g_(i)=Bn_(i) ², where n_(i) is the intrinsiccarrier concentration of the material. From (3) one can see that higherdoping in the p region and/or n region will result in decreased valuesof τ_(p), and therefore greater possibilities for photoconductive gain.The times t_(eave) and t_(have) will depend on the transit times forelectrons and holes to travel through the intrinsic region of the diodeand on the average position in the intrinsic region that photogeneratedEHPs (that are collected by the photodiode) are generated. To firstorder, the photoconductive gain of a p-i-n photodiode for whicht_(eave)<t_(have) may be written as

g=I _(e) /W _(i) +I _(h) /W _(i), for τ_(p) >>t _(have) −t _(eave)  (4a)

g=I _(e) /W _(i) +I _(h) /W _(i)+(t _(have) −t _(eave))/(τ_(p) +t_(tran,e)), for τ_(p) ≦t _(have) −t _(eave)  (4b)

and for a photodiode with t_(eave)>t_(have),

g=I _(e) /W _(i) +I _(h) /W _(i), for τ_(n) >>t _(eave) −t _(have)  (5a)

g=I _(e) /W _(i) +I _(h) /W _(i)+(t _(eave) −t _(have))/(τ_(n) +t_(tran,h)), for τ_(n) ≦t _(eave) −t _(have)  (5b)

where I_(e) and I_(h) are the distances in the intrinsic region of thephotodiode travelled by the photogenerated electron and hole, W_(i) isthe width of the intrinsic region, and t_(tran,e) and t_(tran,h) are thetransit times for an electron and hole to traverse the entire intrinsicregion. These expressions are somewhat simplistic, based on mean timesrather than on the probability distributions of those times.Nonetheless, they are useful for predicting general behavior, and haveproved reasonably accurate in comparison with the few availableexperimental results to date (as shown in FIG. 2).

In bulk p-i-n photodetectors, t_(eave) and t_(have) are typicallyseveral orders of magnitude smaller than τ_(p), so thatτ_(p)>>t_(have)−t_(eave). Thus a typical bulk p-i-n photodetector witht_(eave)<t_(have) will not exhibit photoconductive gain, because thep-region cannot provide excess minority electrons on the time scalerequired. Similarly, a typical bulk p-i-n photodetector witht_(eave)>t_(have) will not exhibit photoconductive gain because then-region cannot provide excess minority holes on the time scalerequired. Therefore, as expected, for typical bulk p-i-n photodetectorsthe traditional assumption of unity photoconductive gain, g=1, is valid.From equations (1) and (2), it may readily be seen that in order toobtain photoconductive gain in a photodiode, one must either delay thephotogenerated carriers in the intrinsic region, so as to increase|t_(have)−t_(eave)|; or speed up the EHP generation in the p-region orn-region, so as to decrease τ_(p) or τ_(n); or both. These approachesare considered in the sections below.

Photogenerated Carrier Delay

Photogenerated carrier delay is the simplest means of attainingphotoconductive gain in a photodiode, but has the disadvantage thatphotodiode bandwidth (speed) will be significantly compromised. Byinserting delay structures in the intrinsic region, |t_(have)−t_(eave)|may be dramatically increased, up to a theoretical limit of the carrierlifetime in the intrinsic region, τ_(i). The delay structures aredesigned to avoid severe trapping of both the photogenerated electronand hole, effectively decreasing the efficiency of EHP collection in thephotodiode, reducing the gain of the photodiode. The preferred delaystructures that are inserted or embedded in the p-n or p-i-n photodiodestructures are those that preferentially delay either the electrons orthe holes, so as to maximize |t_(have)−t_(eave)|. Non-limiting exemplarydelay structures include, heterojunction barriers and wells, resonanttunneling barriers, and graded heterojunctions; the well known“staircase” graded heterojunction structures used in some avalanchephotodiode designs are particularly preferred, as they can be used toimpede one carrier type while allowing the other to be freely swept“down the staircase”. The implementation of these and other delaystructures will be known to those skilled in the art.

FIG. 2 shows the photoconductive gain measured for three InGaAs/InPmultiple quantum well p-i-n diodes for various reverse biases. In orderto obtain these photoconductive gains, the wavelength dependentresponsivity of the diodes in the region of the absorption edge wasfirst measured. The magnitude of the ground state excitonic absorptionpeaks were then compared with the predictions of a theoretical modelthat has been established as accurate using other experimental results(for example, only 3.9% deviation from the InGaAs/InP results reportedin M. Sugawara, T. Fujii, S. Yamazaki, and K. Nakajima, “Opticalcharacteristics of excitons in In_(1−x)Ga_(x)As_(y)P_(1−y)/InP quantumwells”, Phys. Rev. B, 44:1782-1791, 1991), but which assumes the devicesto have no photoconductive gain. The results of this model were fit tothe experimental results using photoconductive gain, g, as a fittingparameter to extract the gains.

In the devices studied, a simple multiple quantum well structure of 20wells with 7.79 nm well widths and 15.04 nm barrier widths wassufficient to obtain modest photoconductive gains with doping levels inthe InP p region of roughly 2.5×10₁₈ cm⁻³. Established tunneling andthermionic emission models were used to calculate the delay effects ofthe multiple quantum well structure. Based on these calculations, and onthe photoconductive gain theory proposed in brief above, the gains inthe three devices were accounted for with reasonable accuracy, as shownby the theoretical curves in FIG. 2.

Increased EHP Generation Rate

The most attractive approach to attaining photoconductive gain in aphotodiode is to increase the rate at which EHPs are generated in thep-region or n-region, so as to decrease τ_(p) or τ_(n). The benefits tothis approach over the photogenerated carrier delay approach are thathigher gains are possible (the delay approach is limited in that|t_(h)−t_(e)| may not be increased beyond τ_(i)) and that this approachdoes not compromise photodiode speed.

The simplest means of decreasing τ_(p) or τ_(n) is to include a regionof “ultra-fast” high B-coefficient material in or near either thep-region or n-region, which has a much lower value of τ_(p) or τ_(n) fora given doping level. Typical B coefficient values for bulk InP is3.3×10⁻¹⁰ cm³s⁻¹ (see R. K. Ahrenkiel, Minority-Carrier Lifetime in InP;pp. 77-79, Properties Of InP, Published by INSPEC, Institution OfElectrical Engineers, London, England, 1991). The helium plasma grownInGaAsP and low temperature grown AIGaAs material systems are excellentultra-fast material prospects, each being demonstrated withrecombination times on the order of a few ps or less. High B-coefficientmaterial may also be engineered by incorporating heterojunctionstructures, such as multiple quantum well structures, within thematerial. Picken and David have proposed that the principle mechanismfor carrier recombination in III-V quantum well structures is throughnonradiative centers associated with the well-barrier interfaces,resulting in significantly higher recombination rates (W. Picken and J.P. R. David, “Carrier decay in GaAs quantum wells”, AppI. Phys. Left.,56:268-270, 1990).

In another embodiment of the invention optical stimulation of a regionin or near the p-region or n-region may be used to enhance the EHPgeneration rates, although care must be taken to ensure such opticalstimulation does not interfere with the interpretation of opticalsignals input to the photodetector. For example, a light source may bemounted above the photodetector with the light focussed onto a region ofthe photodiode to provide a higher photogenerated hole-electron pairconcentrations in that region.

Referring to FIGS. 3 to 5, different embodiments of photodiodes withphotoconductive gain constructed in accordance with the presentinvention are shown. FIG. 3 shows a photodiode 40 comprising a p-typesemiconductor material 42 and an n-type material 44 with electricalcontacts 46 at each end of the photodiode. The depletion region 48 whichforms naturally in the vicinity of the p-n junction will vary dependingon the bias applied across the device. High EHP generation rate regions49 may be embedded in the p-type material 42 and/or the n-type material44 as shown, preferably near the electrical contacts 46. The extents ofthe high EHP generation rate regions 49 are depicted in FIG. 3 as beingrelatively small, but may be of various sizes, potentially including theentire n and p-type regions respectively. FIG. 3 shows a high EHP regionin both p- and n-type regions but only one is required.

Referring to FIG. 4, a photodiode comprising a PIN photodiode structureis shown generally at 50. Photodiode 50 comprises a region of relativelyintrinsic semiconductor material 52 sandwiched between p-typesemiconductor material 42 and n-type material 44 with electricalcontacts 46 at each end of the photodiode. High EHP generation rateregions may be placed in the p-type material 42 and/or the n-typematerial 44 as shown, preferably near the electrical contacts 46. Theextents of the high EHP generation rate regions 49 are depicted in FIG.4 as being relatively small, but may be of various sizes, potentiallyincluding the entire n and p-type regions respectively. FIG. 4 shows ahigh EHP generation rate region in both p- and n-type regions but onlyone is required.

FIG. 5 illustrates another embodiment of a photodiode device 60exhibiting photoconductive gain. Photodiode 60 comprises a PINIPstructure including two p-type regions 42 at the ends with an n-typeregion 44 in the middle and two intrinsic regions 52 sandwiched betweenthe n-type region and in turn sandwiched between the p-type regions. Thehigh EHP generation rate regions 49 may be located in the n-type regionand/or p-type regions. FIG. 5 shows high EHP generation rate regions ina variety of locations but a single region or a combination of severalregions may be used. The extents of the high EHP generation rate regions49 are depicted in FIG. 5 as being relatively small, but may be ofvarious sizes, potentially including the entire n and p-type regionsrespectively.

Combined Approach

In practice, the best design of a photodiode with photoconductive gainwill likely combine the use of delay elements in the intrinsic (ordepletion) region and enhanced EHP generation rates in the p- orn-regions of the photodiode. Given a certain bandwidth (speed) that thephotodiode is required. to operate at, a delay element can be designedsuch that the slow carrier is collected just fast enough to meet thebandwidth specification. The p- or n-region of the photodiode can thenbe engineered to have a given effective τ_(p) or τ_(n), depending on thegain desired for the photodiode. This approach will allow the highestachievable photoconductive gains for a given bandwidth specification.

The basic principles of photoconductive gain in p-i-n photodiodes havebeen introduced, and techniques for designing practical photodiodes thatmake use of these gain effects have been described.

It will be understood by those skilled in the art that while the presentinvention has been illustrated with respect to simple photodiodes, morecomplex devices may be constructed that employ the same principles. Thephotodiodes shown in FIGS. 3 to 5 may be incorporate into standardphotodetection circuits to produce photodetector devices withphotoconductive gain.

The foregoing description of the preferred embodiments of the inventionhas been presented to illustrate the principles of the invention and notto limit the invention to the particular embodiment illustrated. It isintended that the scope of the invention be defined by all of theembodiments encompassed within the following claims and theirequivalents.

I claim:
 1. a photodiode with photoconductive gain, comprising: asemiconductor including at least one p-type region, at least one n-typeregion, and electrical contacts attached to said n-type region and tosaid p-type region, whereby illuminating said semiconductor generateselectron-hole pairs in at least one depletion region formed at a p-njunction; and means for enhancing an electron-hole pair generation ratein at least a portion of said semiconductor outside said at least onedepletion region.
 2. The photodiode device according to claim 1 whereinsaid means for enhancing an electron-hole pair generation rate comprisessaid at least a portion of said semiconductor including an effectivesemiconductor material having a minority carrier lifetime shorter thanor on an order of a difference between the average times for an electronand a hole photogenerated within a specific depletion region to escapesaid specific depletion region.
 3. The photodiode according to claim 2wherein said effective semiconductor material is located within one ofsaid p-type and n-type regions or both.
 4. The photodiode deviceaccording to claim 2 wherein said effective semiconductor materialincludes material having a high B coefficient, wherein B is acharacteristic coefficient for a given semiconductor material related toa thermal generation rate g_(i)=Bn_(i) ², wherein n_(i) is an intrinsiccarrier concentration.
 5. The photodiode device according to claim 2wherein said effective semiconductor material includes material selectedfrom the group consisting of low temperature grown AlGaAs/GaAs materialsand helium plasma grown InGaAsP/InP materials.
 6. The photodiode deviceaccording to claim 2 wherein said effective semiconductor includes aheterojunction structure having one or more abrupt heterojunctions orgraded heterojunctions.
 7. The photodiode device according to claim 1wherein said means for enhancing an electron-hole pair generation ratein at least a portion of said semiconductor outside said at least onedepletion region includes optical stimulation of said at least a portionof said semiconductor.
 8. The photodiode device according to claim 2including means for delaying transport of charge carriers locatedsubstantially in said depletion region, said charge carriers beingselected from the group consisting of electrons and holes.
 9. Thephotodiode device according to claim 8 wherein said means for delayingtransport of charge carriers preferentially delays one of said chargecarriers relative to the other.
 10. The photodiode device according toclaim 8 wherein said means for delaying transport of electrons or holescomprises at least one heterojunction structure.
 11. The photodiodedevice device according to claim 10 wherein said heterojunctionstructure is selected from the group consisting of one or more abruptheterojunctions, one or more graded heterojunctions and any combinationof abrupt heterojunctions and graded heterojunctions.
 12. Thesemiconductor device according to claim 10 wherein said heterojunctionstructures include quantum well structures.
 13. A photodiode withphotoconductive gain, comprising: a semiconductor including at least onep-type region, at least one n-type region, at least one intrinsicregion, and electrical contacts attached to said n-type region and tosaid p-type region, whereby illuminating said semiconductor generateselectron-hole pairs in at least one depletion region formed at a p-njunction and/or at an intrinsic region; and means for enhancing anelectron-hole pair generation rate in at least a portion of saidsemiconductor outside said at least one depletion region.
 14. Thephotodiode device according to claim 13 wherein said means for enhancingan electron-hole pair generation rate comprises said at least a portionof said semiconductor including an effective semiconductor materialhaving a minority carrier lifetime shorter than or on an order of adifference between the average times for an electron and a holephotogenerated within a specific depletion region to escape saidspecific depletion region.
 15. The photodiode according to claim 14wherein said effective semiconductor material is located within one ofsaid p-type and n-type regions or both.
 16. The photodiode deviceaccording to claim 14 wherein said effective semiconductor materialincludes a material having a high B coefficient, wherein B is acharacteristic coefficient for a given semiconductor material related toa thermal generation rate g_(i)=Bn_(i) ², wherein n_(i) is an intrinsiccarrier concentration.
 17. The photodiode device according to claim 14wherein said effective semiconductor material includes material selectedfrom the group consisting of low temperature grown AlGaAs/GaAs materialsand helium plasma grown InGaAsP/InP materials.
 18. The photodiode deviceaccording to claim 15 wherein said effective semiconductor materialincludes a heterojunction structure having one or more abruptheterojunctions or graded heterojunctions.
 19. The photodiode deviceaccording to claim 13 wherein said means for enhancing an electron-holepair generation rate in at least a portion of said semiconductor outsidesaid at least one depletion region includes optical stimulation of saidat least a portion of said semiconductor.
 20. The photodiode deviceaccording to claim 14 including means for delaying transport of chargecarriers located substantially in said depletion region, said chargecarriers being selected from the group consisting of electrons andholes.
 21. The photodiode device according to claim 20 wherein saidmeans for delaying transport of charge carriers preferentially delaysone of said charge carriers relative to the other.
 22. The photodiodedevice according to claim 20 wherein said means for delaying transportof electrons or holes comprises at least one heterojunction structure.23. The photodiode device device according to claim 22 wherein saidheterojunction structure is selected from the group consisting of one ormore abrupt heterojunctions, one or more graded heterojunctions and anycombination of abrupt heterojunctions and graded heterojunctions. 24.The semiconductor device according to claim 22 wherein saidheterojunction structures include quantum well structures.